Hybrid imaging system

ABSTRACT

A hybrid system for digitally screening black and white and/or color images using a number of imaging techniques is disclosed. A hybrid rendering system may be employed to improve image quality by rendering particular image areas according different halftoning schemes according to gray level.

The present application is a continuation-in-part of application Ser.No. 08/625,324 filed on Apr. 1, 1996, also assigned to XeroxCorporation.

The present invention relates to a digitized hybrid imaging system asmay be used in black and white or color printing systems (such as inelectrophotographic printers and copiers), and more particularly, to anapparatus and method for improving output image quality according to theuse of a plurality of imaging techniques in halftoning black and whiteand/or color documents.

In the operation of a copier or printer, particularly color machines, itis highly desirable to have means for processing and enhancing text andimage quality (hereinafter referred to as "image quality" or the likeunless otherwise noted). Particularly in the case of single ormulti-pass color printers, it is highly desirable that an imageprocessing system be employed to reduce imaging problems caused byhalftoning systems not suited to a variety of image types. Likewise,certain image processing systems may be more successfully employed inparticular printer hardware situations. While the present invention isquite suitable for use on the Xerox 4900 family of printers in whichaspects of it have been tested, it may be likewise highly useful with avariety of other xerographic as well as non-xerographic printingsystems.

In the process of digital electrostatographic printing, an electrostaticcharge pattern or latent image corresponding to an original orelectronic document may be produced by a raster output scanner on aninsulating medium. A viewable record is then produced by developing thelatent image with particles of granulated material to form a powderimage thereof. Thereafter, the visible powder image is fused to theinsulating medium, or transferred to a suitable support material andfused thereto. Development of the latent image is achieved by bringing adeveloper mix into contact therewith. Typical developer mixes generallycomprise dyed or colored thermoplastic particles of granulated materialknown in the art as toner particles, which are mixed with carriergranules, such as ferromagnetic granules. When appropriate, tonerparticles are mixed with carrier granules and the toner particles arecharged triboelectrically to the correct polarity. As the developer mixis brought into contact with the electrostatic latent image, the tonerparticles adhere thereto. However, as toner particles are depleted fromthe developer mix, additional toner particles must be supplied. Imagingsystems may be more or less successful in printing high quality imagesof varying types in electrostatographic systems which may have outputcapabilities or efficiencies unlike those found in ink jet or othersystems.

Various systems have been employed to include those set forth in thefollowing disclosures which may be relevant to various aspects of thehybrid imaging systems of the present invention:

U.S. Pat. No. 5,477,305

Patentee: Parker et al.

Issued: Dec. 19, 1995

U.S. Pat. No. 5,341,228

Patentee: Parker et al.

Issued: Aug. 23, 1994

U.S. Pat. No. 5,323,247

Patentee: Parker et al.

Issued: Jun. 21, 1994

U.S. Pat. No. 5,321,525

Patentee: Hains

Issued: Jun. 14, 1994

U.S. Pat. No. 5,291,296

Patentee: Hains

Issued: Mar. 1, 1994

U.S. Pat. No. 5,111,310

Patentee: Parker et al.

Issued: May 5, 1992

U.S. Pat. No. 4,955,065

Patentee: Ulichney

Issued: Sep. 4, 1990

U.S. Pat. No. 4,736,254

Patentee: Kotera et al.

Issued: Apr. 5, 1988

U.S. Pat. No. 4,698,691

Patentee: Suzuki et al.

Issued: Oct. 6, 1987

U.S. Pat. No. 4,245,258

Patentee: Holladay

Issued: Jan. 13, 1991.

"Dithering with Blue Noise" by Robert A. Ulichney. Proceedings of theIEE, Vol. 76, No. 1, January 1988. Pages 56-79. "Modified Approach tothe Construction of a Blue Noise Mask" by Meng Yao and Kevin J. Parkerof the University of Rochester. Journal of Electronic Imaging, January1994, Vol. 3(1). Pages 92-97. "Digital Halftoning Using a Blue NoiseMask" by Theophano Mista and Kevin J. Parker of the University ofRochester. SPIE Vol. 1452 Image Processing Algorithms and Techniques 11(1991). Pages 47-56.

U.S. Pat. No. 5,447,305 teaches a method of and system for rendering ahalftone image of a gray scale image by utilizing a pixel-by-pixelcomparison of the gray scale image against a blue noise mask disclosedin which the gray scale image is scanned on a pixel-by-pixel basis andcompared on a pixel-by-pixel basis to an array of corresponding datapoints contained in a blue noise mask. Multiple masks may be used tohalftone color images. Modifications can be made by a user to improvemask performance.

U.S. Pat. No. 5,341,228 teaches a method of and system for rendering ahalftone image of a gray scale image by utilizing a pixel-by-pixelcomparison of the gray scale image against a blue noise mask disclosedin which the gray scale image is scanned on a pixel-by-pixel basis andcompared on a pixel-by-pixel basis to an array of corresponding datapoints contained in a blue noise mask stored in a PROM or computermemory in order to produce the desired halftoned image.

U.S. Pat. No. 5,323,247 also disclosed a method of and system forrendering a halftone image of a gray scale image by utilizing apixel-by-pixel comparison of the gray scale image against a blue noisemask in which the gray scale image is scanned on a pixel-by-pixel basisand compared on a pixel-by-pixel basis to an array of corresponding datapoints contained in a blue noise mask stored in a PROM or computermemory in order to produce the desired halftoned image.

U.S. Pat. No. 5,321,525 discloses a method of quantizing pixel values inan image formed by a plurality of pixels, each pixel representing anoptical density of the image at a location within the image, and havingan original optical density value selected from one of a set of `c`original optical density values that has a number of members larger thana desired output set of `d` optical density values through a process ofcombined halftoning and cell-to-cell error diffusion.

U.S. Pat. No. 5,291,296 discloses a method of halftoning according to a"quad dot" system, and is also referred to below.

U.S. Pat. No. 5,111,310 discloses a method of and system for rendering ahalftone image of a gray scale image by utilizing a pixel-by-pixelcomparison of the gray scale image against a blue noise mask in whichthe gray scale image is scanned on a pixel-by-pixel basis and comparedon a pixel-by-pixel basis to an array of corresponding data pointscontained in a blue noise mask stored in a PROM or computer memory inorder to produce the desired halftoned image.

U.S. Pat. No. 4,955,065 discloses a digital image processing system forconverting continuous tone pixel values representing an image intohalftone or dithered pixel values, with the dithered pixel valuesrepresenting each pixel having fewer bits than are used to representeach pixel in the continuous tone image.

U.S. Pat. No. 4,736,254 discloses a halftone signal having one of twodiscrete levels is generated for each print position along each printline by comparison between a gray scale value of an original with athreshold value stored in a memory. The memory having a matrix array ofcells each storing a particular threshold value where M, N, a and β areintegers.

U.S. Pat. No. 4,698,691 discloses a halftone image processing method forproviding image information in a bit distribution by specifying a matrixpattern in response to tone data which is indicative of a recordingdensity. Several matrix pattern groups which are prepared eachcomprising matrix patterns which are larger in number than dots whichdefine a dot matrix.

U.S. Pat. No. 4,245,258 discloses an electrical screening system forbinary displays or binary graphic recording systems which suppressesfalse contours. The suppression is achieved by increasing the number ofgray levels that a given m×n matrix of pixels can represent.

The article "Dithering with Blue Noise" describes and compares imageprocessing systems employing blue noise with error diffusion and otheroutputs.

Digital halftoning processes and desirable characteristics are comparedand summarized; optimized blue noise generations are explained anddemonstrated.

The article "Modified Approach to the Construction of a Blue Noise Mask"teaches a modified method of and system for rendering a halftone imageof a gray scale image by utilizing a pixel-by-pixel comparison of thegray scale image against a blue noise mask. Steps to produce improvedmasks are explained.

The article "Digital halftoning Using a blue Noise Mask" likewiseteaches earlier methods by Mista and Parker for rendering a halftoneimages of a gray scale utilizing a pixel-by-pixel comparison of the grayscale image against a blue noise mask.

In accordance with one aspect of the present invention, there isprovided a system for digitally halftoning images according to aplurality of imaging techniques. The system may include a compositescreen for halftoning a digitized image constructed according to thefollowing steps:

selecting a first halftoning system of a first type having a first setof pixel placement characteristics; selecting a second halftoning systemof a second type having a second set of pixel placement characteristics;determining a gray scale transition region for transitioning from thefirst halftoning system to the second halftoning system; and merging thefirst halftoning system with the second halftoning system so as toprovide a hybrid halftoning system in the gray scale transition region,the hybrid halftoning system including a hybrid set of pixelcharacteristics of the first set of pixel placement characteristics andthe second set of pixel placement characteristics.

Other features of the present invention will become apparent as thefollowing description proceeds and upon reference to the drawings, inwhich:

FIG. 1 is a flowchart showing a hybrid dot screening system of thepresent invention;

FIG. 2 is a flowchart showing the exemplary use of multiple imagescreening techniques in a system of the present invention;

FIG. 3 is a representative gray level rendering option menu screen ofthe present invention;

FIG. 4 is flow chart of an embodiment of the present invention; and

FIG. 5 is a schematic elevational view showing an exemplary colorxerographic printing machine and networked PC incorporating features ofthe present invention therein.

While the present invention will hereinafter be described in connectionwith preferred embodiments thereof, it will be understood that it is notintended to limit the invention to these embodiments. On the contrary,it is intended to cover all alternatives, modifications and equivalents,as may be included within the spirit and scope of the invention asdefined by the appended claims.

For a general understanding of the features of the present invention,reference is made to the drawings. FIG. 5 is a schematic elevationalview showing an exemplary electrophotographic printing/copying machineand a networked PC which may incorporate features of the presentinvention therein. It will become evident from the following discussionthat the system of the present invention is equally well suited for usein a wide variety of printing and copying systems, and therefore is notlimited in application to the particular system(s) shown and describedherein.

To begin by way of general explanation, FIG. 5 is a schematicelevational view showing an electrophotographic printing machine andnetworked PC which may incorporate features of the present inventiontherein. An image processing station (IPS), indicated generally by thereference numeral 12, contains data processing and control electronicswhich prepare and manage the image data flow to a raster output scanner(ROS), indicated generally by the reference numeral 16. A network of oneor more personal computers (PC), indicated generally by the referencenumeral 5, is shown interfacing/in communication with IPS 12. A userinterface (U1), indicated generally by the reference numeral 14, is alsoin communication with IPS 12.

U1 14 enables an operator to control and monitor various operatoradjustable functions and maintenance activities. The operator actuatesthe appropriate keys of U1 14 to adjust the parameters of the copy. U114 may be a touch screen, or any other suitable control panel, providingan operator interface with the system. The output signal from U1 14 istransmitted to IPS 12. U1 14 may also display electronic documents on adisplay screen (not shown in FIG. 17), as well as carry out the systemof the present invention as described in association with FIGS. 2through 4 below.

As further shown in FIG. 5, a multiple color original document 38 may bepositioned on (optional) raster input scanner (RIS), indicated generallyby the reference numeral 10. The RIS contains document illuminationlamps, optics, a mechanical scanning drive, and a charge coupled device(CCD array) or full width color scanning array. RIS 10 captures theentire image from original document 38 and converts it to a series ofraster scan lines and moreover measures a set of primary colordensities, i.e., red, green and blue densities, at each point of theoriginal document. RIS 10 may provide data on the scanned image to IPS12, indirectly to PC 5 and/or directly to PC 5.

Digitized electronic documents may be created, screened, modified,stored and/or otherwise processed by PC 5 prior to transmission/relay toIPS 12 for printing on printer 18. The display of PC 5 may showelectronic documents on a screen (not shown in FIG. 5). IPS 12 mayinclude the processor(s) and controller(s) (not shown in FIG. 5)required to perform the system of the present invention.

IPS 12 also may transmits signals corresponding to the desiredelectronic or scanned image to ROS 16, which creates the output copyimage. ROS 16 includes a laser with rotating polygon mirror blocks. TheROS illuminates, via mirror 37, the charged portion of a photoconductivebelt 20 of a printer or marking engine, indicated generally by thereference numeral 18, at a rate of about 400 pixels per inch, to achievea set of subtractive primary latent images. (Other implementations mayinclude other pixel resolutions of varying types 600×600 dpi, or evenasymmetrical resolutions, such as 300×1200 dpi, both configurations ofwhich are employed in versions of the Xerox 4900 printer.) The ROS willexpose the photoconductive belt to record three or four latent imageswhich correspond to the signals transmitted from IPS 12. One latentimage is developed with cyan developer material. Another latent image isdeveloped with magenta developer material and the third latent image isdeveloped with yellow developer material. A black latent image may bedeveloped in lieu of or in addition to other (colored) latent images.These developed images are transferred to a copy sheet in superimposedregistration with one another to form a multicolored image on the copysheet. This multicolored image is then fused to the copy sheet forming acolor copy.

With continued reference to FIG. 5, printer or marking engine 18 is anelectrophotographic printing machine. Photoconductive belt 20 of markingengine 18 is preferably made from a photoconductive material. Thephotoconductive belt moves in the direction of arrow 22 to advancesuccessive portions of the photoconductive surface sequentially throughthe various processing stations disposed about the path of movementthereof. Photoconductive belt 20 is entrained about rollers 23 and 26,tensioning roller 28, and drive roller 30. Drive roller 30 is rotated bya motor 32 coupled thereto by suitable means such as a belt drive. Asroller 30 rotates, it advances belt 20 in the direction of arrow 22.

Initially, a portion of photoconductive belt 20 passes through acharging station, indicated generally by the reference numeral 33. Atcharging station 33, a corona generating device 34 chargesphotoconductive belt 20 to a relatively high, substantially uniformpotential.

Next, the charged photoconductive surface is rotated to an exposurestation, indicated generally by the reference numeral 35. Exposurestation 35 receives a modulated light beam corresponding to informationderived by RIS 10 having multicolored original document 38 positionedthereat. The modulated light beam impinges on the surface ofphotoconductive belt 20. The beam illuminates the charged portion of thephotoconductive belt to form an electrostatic latent image. Thephotoconductive belt is exposed three or four times to record three orfour latent images thereon.

After the electrostatic latent images have been recorded onphotoconductive belt 20, the belt advances such latent images to adevelopment station, indicated generally by the reference numeral 39.The development station includes four individual developer unitsindicated by reference numerals 40, 42, 44 and 46. The developer unitsare of a type generally referred to in the art as "magnetic brushdevelopment units." Typically, a magnetic brush development systememploys a magnetizable developer material including magnetic carriergranules having toner particles adhering triboelectrically thereto. Thedeveloper material is continually brought through a directional fluxfield to form a brush of developer material. The developer material isconstantly moving so as to continually provide the brush with freshdeveloper material. Development is achieved by bringing the brush ofdeveloper material into contact with the photoconductive surface.Developer units 40, 42, and 44, respectively, apply toner particles of aspecific color which corresponds to the complement of the specific colorseparated electrostatic latent image recorded on the photoconductivesurface.

The color of each of the toner particles is adapted to absorb lightwithin a preselected spectral region of the electromagnetic wavespectrum. For example, an electrostatic latent image formed bydischarging the portions of charge on the photoconductive beltcorresponding to the green regions of the original document will recordthe red and blue portions as areas of relatively high charge density onphotoconductive belt 20, while the green areas will be reduced to avoltage level ineffective for development. The charged areas are thenmade visible by having developer unit 40 apply green absorbing (magenta)toner particles onto the electrostatic latent image recorded onphotoconductive belt 20. Similarly, a blue separation is developed bydeveloper unit 42 with blue absorbing (yellow) toner particles, whilethe red separation is developed by developer unit 44 with red absorbing(cyan) toner particles. Developer unit 46 contains black toner particlesand may be used to develop the electrostatic latent image formed from ablack and white original document. Each of the developer units is movedinto and out of an operative position. In the operative position, themagnetic brush is substantially adjacent the photoconductive belt, whilein the nonoperative position, the magnetic brush is spaced therefrom.During development of each electrostatic latent image, only onedeveloper unit is in the operative position, the remaining developerunits are in the nonoperative position.

After development, the toner image is moved to a transfer station,indicated generally by the reference numeral 65. Transfer station 65includes a transfer zone, generally indicated by reference numeral 64.In transfer zone 64, the toner image is transferred to a sheet ofsupport material, such as plain paper amongst others. At transferstation 65, a sheet transport apparatus, indicated generally by thereference numeral 48, moves the sheet into contact with photoconductivebelt 20. Sheet transport 48 has a pair of spaced belts 54 entrainedabout a pair of substantially cylindrical rollers 50 and 53. A sheetgripper (not shown in FIG. 5) extends between belts 54 and moves inunison therewith. A sheet 25 is advanced from a stack of sheets 56disposed on a tray. A friction retard feeder 58 advances the uppermostsheet from stack 56 onto a pre-transfer transport 60. Transport 60advances the sheet (not shown in FIG. 5) to sheet transport 48. Thesheet is advanced by transport 60 in synchronism with the movement ofthe sheet gripper. The sheet gripper then closes securing the sheetthereto for movement therewith in a recirculating path. The leading edgeof the sheet (again, not shown in FIG. 5) is secured releasably by thesheet gripper. As belts 54 move in the direction of arrow 62, the sheetmoves into contact with the photoconductive belt, in synchronism withthe toner image developed thereon. In transfer zone 64, a coronagenerating device 66 sprays ions onto the backside of the sheet so as tocharge the sheet to the proper magnitude and polarity for attracting thetoner image from photoconductive belt 20 thereto. The sheet remainssecured to the sheet gripper so as to move in a recirculating path forthree cycles. In this way, three or four different color toner imagesare transferred to the sheet in superimposed registration with oneanother.

One skilled in the art will appreciate that the sheet may move in arecirculating path for four cycles when under color black removal isused. Each of the electrostatic latent images recorded on thephotoconductive surface is developed with the appropriately coloredtoner and transferred, in superimposed registration with one another, tothe sheet to form the multicolored copy of the colored originaldocument. After the last transfer operation, the sheet transport systemdirects the sheet to a vacuum conveyor 68. Vacuum conveyor 68 transportsthe sheet, in the direction of arrow 70, to a fusing station, indicatedgenerally by the reference numeral 71, where the transferred toner imageis permanently fused to the sheet. Thereafter, the sheet is advanced bya pair of rolls 76 to a catch tray 78 for subsequent removal therefromby the machine operator.

The final processing station in the direction of movement of belt 20, asindicated by arrow 22, is a photoreceptor cleaning apparatus, indicatedgenerally by the reference numeral 73. A rotatably mounted fibrous brush72 may be positioned in the cleaning station and maintained in contactwith photoconductive belt 20 to remove residual toner particlesremaining after the transfer operation. Thereafter, lamp 82 illuminatesphotoconductive belt 20 to remove any residual charge remaining thereonprior to the start of the next successive cycle. As mentioned above,other xerographic and non-xerographic printer hardware implementationsmay be used with the hybrid imaging systems of the present invention,such as in the case of versions of the Xerox 4900 printer (which employsan intermediate transfer system) in which certain aspects of the systemas outlined below have been tested.

FIG. 1 shows a system for halftoning gray scale black and white or colorimages which utilizes pixel-by-pixel comparison of the image against aordered hybrid dot screen. The system includes the use of a ordered dotmatrix look-up table or thresholding system (such as a 2×2 ordered dotmatrix), wherein each quadrant of the matrix is always filled in aparticular order. For example, in an up to a 25% "fill" of a gray scalearea printed output, the first designated quadrant of the matrix in acontinuous halftone area will always be used or filled before the secondordered quadrant is utilized. Within each of the gray scale quadranthalftoning ranges (0-25%, 26-50%; 51-75%; and 76-100%), halftoning maybe accomplished using a variety of stochastic screening, thresholding,dithering, randomized dot systems (such as blue noise-emulatingfunctions) or other compartmentally useful imaging techniques. While thedeterministic nature of such an ordered dot system may not work well onsome image types or with some imaging situations or hardwareimplementations (such as by resulting in "checkerboard" effects at thetransition regions), such a system may be employed in many situationswith good to excellent results.

FIG. 1 shows the hybrid "matrix" dot screen system in which a stochasticscreen function of the dimensions M×N for use in selected instances(such as 128×128). Thereafter a list of x, y coordinates sorted in thematrix in the order in which they turn on is made. The scale of thematrix P×Q may preferably be linearly translated into the newcoordinates. For a 2×2 matrix, the function is scaled according to thedimension shown in FIG. 1 of M' and N' and the coordinates of x' and y'.Thereafter a list for x" and y" is created for all x', y' coordinates;this operation is repeated P^(*) Q (four times in the illustratedexample) to create a matrix of the desired matrix dot size. Thereafter,the matrix is projected into a new array. At the same time, before orafter the stochastic screen generation, the matrix dot scaling stepsoccur. Preferably a list of x, y coordinates for sorting thresholds atthe coordinate 0,0; 1,1; 0,1; 1,0 are created. A counter sorted list ofx, y coordinates for the matrix is thereafter generated. Again, theseoperations may preferably be performed linearly so as to create a moreefficient system for generating the hybrid dot. Thereafter, an orderedlist of the size (P×Q)*(M×N) that is (2×2)*(128×128) is generatedresulting in 65,536 address list lookup table. Pixel-by-pixel comparisonmay thereby be performed on this hybrid dot screen or listed lookuptable. The stochastic screen function may be of a nature to emulate bluenoise or many other systems of generating random screens to fill in dotquadrants may be used, as described above and below.

The absence of low frequency components in the frequency domaincorresponds to the absence of "disturbing artifacts" in the spatialdomain (meaning the actual appearance of the dot profiles when printed).While the hybrid dot system of the present invention will result inordered matrix dot filling, desirable outputs are obtained using thehybrid dot system. The cutoff frequency f_(g), which is termed thePrincipal Frequency, depends as follows on the gray level g: ##EQU1##where R, as before, is the distance between addressable points on thedisplay and the gray level g is normalized between 0 and 1. According tothis formula, f_(g) achieves its maximum value where g=1/2 (50%), sinceat that level the populations of black and white dots are equal and thusvery high frequency components appear in the binary image. It is at thisgray level that would appear the most difficult location to attain dotprofiles without disturbing artifacts.

In one example, a stochastic screen function may be generated accordingto a number of steps proposed in the Article "Modified approach to theconstruction of a blue noise mask":

1. Set the number M of pairs of 1's and 0's to be swapped in eachiteration.

2. Rotate the 1-D filter with anisotropy to make the 2-D filter.

3. Create the initial binary pattern for level g₁ +Δg by convertingrandomly K0's to 1's in the binary pattern for g, (where K=W×w/L, W×W isthe size of the BNM and L is the total number of levels).

4. Take the FFT (fourier transform) of the binary pattern for level g₁+Δg.

5. Filter the current binary pattern with the 2-D filter appropriate forlevel g₁ +Δg.

6. Take the IFFT (inverse fourier transform) of the filtered pattern.

7. Form an error array by computing the difference between the filteredpattern and g₁ +Δg.

8. Sort the errors into two cases:

For the K1's that are in the binary pattern for level g₁ +Δg but not inthe binary pattern for g₁, sort the positive errors.

For the 0's in the binary pattern, sort the negative errors.

9. Swap the M pairs of 1's and 0's that have the highest positive errorsand negative errors.

10. Compute the MSE (mean square error) of the filtered pattern withrespect to the gray level g₁ +Δg. If the MSE drops, go to step 5 andproceed to the next iteration. If the MSE increases but M≠1, reduce M byhalf, go to step 5. Otherwise, go to step 12.

11. Update the mask: ##EQU2## where the bar is the NOT operation. 12. Ifg₁ +Δg<255, let g₁ =g₁ +Δg reset M, and go to step 2.

This further modified approach for the generation of a bluenoise-emulating function can be enhanced by performing additional steps.For example, the dynamic range does not work well in some printerhardware system implementations. The method was not designed to be usedas part of a hybrid dot as required in the present invention. By way offurther example, the aforementioned method does not relate the amount ofresidual low frequency power in executing the error decision whendetermining the swaps made at each level. Lastly, the algorithms may notbe readily adaptable for automatic execution on a computer.

The hybrid dot system preferably includes a modified iterativestochastic screen function generated according to the following steps:

A. Generate a stochastic function with the steps proposed above. (Equalnumbers of pixels are turned on in each step.)

B. Take the L^(*) (luminance or "lightness") measurement of theresultant stochastic function.

C. Invert the measurement curve so that the output L^(*) curve is linearwith respect to digital count.

D. Use the inverted curve to determine the number of pixels to turn onat each level.

E. Generate the first level bitmap as "seed".

F. Starting at level above, calculate the number of pixels to turn on atthe current level according to the inverted L^(*) curve.

G. Use the numbered steps outlined above to identify the locations withhighest DC level and pixel value of 0.

H. For the number of pixels to be turned on at the current level, turnon pixels at locations with highest DC level in a descending order.

I. Sort pixels that is currently off (0) in a descending order of DClevel. Repeat the same procedure on pixels that are turned on (1) atthis level.

J. Swap N pixels on each list

K. If the resultant DC level of the bitmap decreases, repeat step G.Otherwise, divide N by 2 and repeat step G. If N=1 restart loop withhalf of the pixels to begin with. If the starting value of N=2, repeatstep G until the DC level of the bitmap reaches a steady state.

L. Take the FFT; look for maximum DC levels within transform range. Lookup number of pixels to be added to the next level from the step Dinverted curve.

M. Add pixels to positions of highest DC value in descending order. Goto step G and repeat.

In this manner, an improved stochastic function can be generated for usein the hybrid dot of the present invention. Several important aspects ofthe improved methods outlined above enable optimization of the functionto be used in the hybrid dot. First by actually measuring luminance on asensing device as set forth in step B, the outputs of the screen can beknown and its performance optimized according to the printing hardware(such as the 4900) that the screen will be used on. Further, steps B-Finvolve a summation operation that by using the inverted curve permitsthe creation of a more linear (consistent) output. Additionally, the DClevels (step K) are placed in a buffer, at which time the repeatabilityof the results can be established, such that "steady state" conditionsmay be identified and checked. Finally, the system employs a repeatingloop logic that permits the levels of the improved screen to be builtautomatically.

FIG. 2 shows a halftoning menu system which in itself can employ animaging system including a further "hybrid" imaging system includingmultiple types of screening or imaging techniques in generating orrendering black and white or color images. For example, black and whiteimage halftoning might be performed such that at level 1, that is, 0 tox' gray levels hybrid dot screening according to the system outlinedabove with regard to FIG. 1 may be performed. With regard to level 2,that is, x' up to 256 gray levels quad dot screening according to U.S.Pat. No. 5,291,296 (incorporated herein by reference) may be used. Byway of further example according to the system set forth in FIG. 2, ifonly 3 color print imaging is employed, certain colors may be halftonedaccording to designated gray levels while other colors are halftonedaccording to a different gray level screening technique. FIG. 2 alsoshows an embodiment in which cyan and magenta halftoning is performed attwo different levels by two different halftoning systems. At level 1,that is, 0 to y', gray level hybrid dot screening may be performed,whereas at gray level 2, that is, y' to 256 gray level, an errordiffusion system is used to halftone these gray levels. As further shownin the example of FIG. 2, for yellow images, halftoning is completed bya single method (hybrid dot screening) for all gray levels. Whenundercolor removal is used (that is, black toner is used to darken theoutput image to the correct level so as to lower colored toner uselevels), the entire FIG. 2 system may be used. The FIG. 2 color imagehalftoning scenarios can be modified in a variety of situations inaccordance with the spirit of the present invention. In the colorimaging breakdown portion suggested, the FIG. 2 example highlights suchconcepts as, for example, that for a lighter color such as yellow, theimaging system may be less critical to the output of the final halftonedimage. For certain (such as darker) colors, the halftoning system usedmay be more critical and have a greater influence over the quality ofthe image generated at particular gray levels.

In accordance with the system described in association with FIG. 2, avariety of modifications are envisioned such that the quality of thefinal rendered composite (1-4 color) halftoned image is maximized.Modifications on the hybrid imaging system of the present invention maybe used to reduce the occurrence of undesirable image artifacts such ascontouring in the highlight regions, noisiness of halftone imagesthrough all gray levels and other undesirable effects may be employed.The hybridized use of multiple screening techniques capitalizes on thefact that certain gray level ranges can mean more desirably halftoningwith a particular screening technique. Finally, in some instances, whena particular color or image type is being gray scaled (such as yellow),the system recognizes that the most efficient (and simplified) grayscale imaging technique can be used without detriment to the imagequality of the final composite image.

The hybrid dot thresholding system of FIG. 1 has been shown to beparticularly useful as an imaging option in four color printers, such asin the Xerox 4900 family of printers. The 4900 or other networkable oras a stand-alone printers or copiers may permit users to select betweenseveral halftoning options. In one embodiment, a 128×128 (N×M) sizestochastic or blue noise emulating function may be used in conjunctionwith the 2×2 ordered dot matrix to achieve a desired output of 256 graylevels in both black and white or color implementations. A linearizedscaled function permits projection into the hybrid dot array accordingto a sorted list of matrix dot coordinates. The resultant method canyield quality dot patterns across the gray scale. A single hybrid dothalftoning system may be used for each color, or different thresholding(hybrid dot or other) systems may be used. While the ordered filling ofthis ordered dot system can result "checkerboard" effects (particularlyat or near the 25%, 50% or 75% dot fill areas), this effect can be quitedesirable in many imaging scenarios, (or other halftoning systems may beused to prevent particularly undesirable outputs that might be generatedas a result of this system as shown in FIG. 2).

In traditional halftone technology, there may often be a tradeoffbetween the use of the standard number of gray levels (256) in a dotprofile and the spatial frequency of the screen. If the resolution ofthe dot was high, then it sacrificed the number of gray levels (i.e.,the number of micro dots in a halftone cell). The most common dot growthpattern is known as a clustered dot. This type of screen grows out fromthe center of the dot as the gray levels increase. Several alternativesto this tradeoff have been employed with varying success. For instance,a matrix dot that is several times larger than a traditional clusterdot, but has multiple centers can be used to achieve a larger number oflevels without sacrificing spatial resolution. The Xerox quad dot (U.S.Pat. No. 5,291,296) discussed above is an example of a multi centereddot with 4 centers and the ability to have 4 times as many levels. Thisdot can significantly improve image quality. There is a limit to thenumber of centers one can design into this dot because low frequencyartifacts become more apparent as the dot grows. Another approach tohalftoning is error diffusion. Here, a gray level is thresholded and theerror between the threshold and the gray level is distributed downstreamto neighboring pixels. This technique is very good at renderingpictorial images, although it exhibits "worm" like structures and iscomputationally intensive to implement. Typically, extra hardware isnecessary to perform error diffusion in a timely manner.

The FIG. 1 system provides desirable halftoning results particularly forpictorial images is stochastic screening. The halftone cell isrelatively large and affords many gray levels, and avoids problems thatcan occur with a large dot by minimizing visual structure within the dotitself. In some scenarios, stochastic screens are used wherein theconstraint to minimize visual structure is to generate high frequencynoise with the principle frequency in a relatively insensitive region ofthe human visual contrast sensitivity function. Such masks typicallyappear to be visually noisier than structured dots, although theincreased latitude in number of levels makes them valuable. Theselective combination of several thresholding techniques can potentiallysolve the problems of low frequency structure of multi centered dot andthe perceived noisiness of stochastic by utilizing several halftoningsystem on a selective basis, depending on gray level thresholds orranges. For example, a quad dot system may be merged in such a way withanother imaging technique that the advantageous properties of bothapproach may be used to create a hybrid significantly better than onesystem or another. The present invention allows different screening orthresholding systems to be selectively used to render image areasaccording to gray level. Further, different object types may also berendered differently. (The term "object" as used herein generally refersto image types including text, graphics and bitmaps, although otherclasses or types of objects may differentiated in other systems.) Forexample, graphic objects often desirably may be output with vivid, boldcolors than other types of images, while photos generally need to haveless dramatic, more natural tones applied to produce a more desirableoutput. A particular desired colorization scheme may be related to thehalftoning system used. The limitations and/or characteristics ofparticular print engines can also dictate which halftoning, screening orthresholding systems are most useful in rendering a multi-objectdocument or image.

FIG. 3 shows an exemplary user interface hybrid halftoning selectionsystem, in which a "Hybrid Imaging System Preset Menu" is used todisplay select the various available gray level rendering options A usercan proceed to display a graphical representation of one or more presetson screen display 100, and then select for implementation a preloadedgray level rendering preset from the available menu selections. GrayScale Level Rendering Preset display portions (shown as "Preset" #1, 2and 3 in FIG. 3) can be used to graphically represent the use of two ormore in which one imaging technique transitions to (or merges with)another, as described in greater detail below. Some of the types ofrendering techniques that may be selected according to the FIG. 3 menuare as follows:

Matrix Dot: Stochastic or random noise dot in an ordered 2×2 matrix(1,3,2 4 order), often useful in a variety of higher density colorimages.

Cluster Dot: Larger cluster of dots providing increased gray levelsoften useful in a variety of higher density color images.

Random or Stochastic Dot: Randomly generated noise dot (without orderedmatrix); useful certain applications such as pictorials and graphics.

Quad Dot: Versatile halftoning method; often best for medium to highdensity images.

White Noise: Generally useful for lower density applications.

Blue Noise: Also generally useful for lower density applications.

Error Diffusion: Versatile halftoning method; often used for non-textimages.

Combinations and variations on these techniques may also be used, suchas are included in selection #1 shown on the of the representative menuscreen of FIG. 3. The descriptions provided above are intended to beexemplary of the type of information to be provided to a user on thedisplay, and while being useful in some situations, may not hold true inmany imaging applications.

In some imaging applications, a single size cluster halftone screensimply does not provide enough simulated gray levels. Larger sized dotcluster systems may result in more pleasing/higher quality renderedimages. The present invention includes a system in which one imagingsystem is transitioned to another imaging system, whereby a renderingsystem (such as clustered quad dot) is effectively merged with a secondimaging system (such as white noise) to provide relatively contour-freetransitioning from one imaging system to another.

In the selection #1 example of FIG. 3, a Cluster Quad Dot renderingsystem is merged with a White Noise rendering system by of the presentinvention. For example, beginning (in a descending manner) at gray level65, pixels are removed from a cluster dot rendering system according toa white noise pattern. In this manner, additional gray levels are addedto the cluster dot rendering system through this merging of onerendering system with another. In this manner, in the transition zone,the two imaging techniques are related related to each other; that is,the transition zone join the two imaging techniques so as to includeaspects or traits of both. By way of example, as pixels are removed froma cluster dot rendering system at gray level 65 according to a whitenoise pattern (Preset #1 of FIG. 3) just below gray level 65, as only afew dots in the clusters are removed, the imaging technique is a closerrelative of the cluster dot rendering system than the white noiserendering system it gradually transitions to. Distinctive andundesirable contouring or "stepping" can be thus be avoided orminimized.

U.S. application Ser. No. 08/753,576 "entitled Cluster Dot HalftoningSystem" filed on the same date as the present application and alsoassigned to Xerox, describes in relevant part an optimization techniquethat can be used to in the transition region described above, and isincorporated herein by reference. This implementation of the system ofthe present invention provides that pixels are turned-on in clusters butthe clusters themselves are "selected" based on a optimized stochasticor randomized screening system. In traditional stochastic screen design,it is the turn-on order of each pixel that is stochastic instead of acluster of pixels. In particular, the screen of the present invention isuseful to eliminate moire patterns as well as to provide for a morestable imaging platform (that is, a process that will be less prone toresulting in undesirable image outputs such as may occur in xerographicor other types of print engines). The present invention thus permits thecombination of two or more imaging systems so as to capitalize on theadvantages and/or minimize the weaknesses of each imaging technique. Avariety of halftone screens may be used in conjunction with the presentinvention, to include the examples described or incorporated herein byreference above.

The FIG. 4 flow chart provides a general description of how a hybridimaging system is assembled composite screen system 100 may be assembledaccording to one embodiment of the present invention (as will later bedescribed in additional detail.) A composite screen for halftoning adigitized image is constructed according to FIG. 4. In a first orpreliminary step, the multiple halftoning systems to be used to create ahybrid or composite screen (such as the "Gray Scale Level RenderingPresets" 1, 2 and 3 shown in FIG. 3). FIG. 4 shows this step as theblock "Select a first halftoning system and a second halftoning system".Next, the gray scale transition region or start point for transitioningfrom the first halftoning system to the second halftoning system must beestablished. This step is shown as the "Define the gray scale transitionregion" block in FIG. 4. In the next step, the first halftoning systemis merged with the second halftoning system so as to provide a hybridhalftoning system in the gray scale transition region. In order toinsure a smooth transition of the first halftoning system having whatmay be called a first set of pixel placement characteristics to thesecond halftoning system having a second set of pixel placementcharacteristics. This step is shown as the "Merge the first and secondhalftoning systems in the transition region" block of FIG. 4. Thetransition region must, as described in the examples above, includecharacteristics or traits that are a hybrid or combination of these twosystems. Thereafter, the image is rendered with the composite or hybridimage system, according to the "Render Image with Hybrid Image System"block of FIG. 4.

While present invention has been described in conjunction with variousembodiments, it is evident that many alternatives, modifications, andvariations will be apparent to those skilled in the art. Accordingly, itis intended to embrace all such alternatives, modifications, andvariations as fall within the spirit and broad scope of the appendedclaims.

I claim:
 1. A method of constructing a composite screen for halftoning adigitized color image having a predetermined number of colorseparations, comprising:selecting a first halftoning system of a firsttype having a first set of pixel placement characteristics; selecting asecond halftoning system of a second type having a second set of pixelplacement characteristics; determining a gray scale transition regionfor transitioning from the first halftoning system to the secondhalftoning system; merging the first halftoning system with the secondhalftoning system so as to provide a hybrid halftoning system in thegray scale transition region, said hybrid halftoning system including ahybrid set of pixel characteristics of said first set of pixel placementcharacteristics and said second set of pixel placement characteristics,and rendering a first number of color separations of a color image usingsaid hybrid halftoning system, the first number of color separationsbeing equal to a value which is less the predetermined number of colorseparations.
 2. The method of claim 1, wherein the gray scale includes256 gray levels.
 3. The method of claim 2, wherein an upper gray scalelimit of the transition region is
 65. 4. The method of claim 1, whereinthe first halftoning system is a cluster dot halftoning system.
 5. Themethod of claim 1, wherein the first halftoning system is a matrix dothalftoning system.
 6. The method of claim 1, wherein the firsthalftoning system is a stochastic dot halftoning system.
 7. The methodof claim 1, wherein an upper gray scale limit of the transition regionis equal to approximately one quarter of a total number of availablegrey levels.
 8. A method of constructing a composite screen forhalftoning a digitized image, comprising:selecting a first halftoningsystem of a first type having a first set of pixel placementcharacteristics; selecting a second halftoning system of a second typehaving a second set of pixel placement characteristics; determining agray scale transition region for transitioning from the first halftoningsystem to the second halftoning system; and merging the first halftoningsystem with the second halftoning system so as to provide a hybridhalftoning system in the gray scale transition region, said hybridhalftoning system including a hybrid set of pixel characteristics ofsaid first set of pixel placement characteristics and said second set ofpixel placement characteristics; selecting a third halftoning system ofa second type having a third set of pixel placement characteristics;determining a second gray scale transition region for transitioning fromthe second halftoning system to the third halftoning system; and mergingthe second halftoning system with the third halftoning system so as toprovide a hybrid halftoning system in the second gray scale transitionregion, said hybrid halftoning system including a second hybrid set ofpixel characteristics of said second set of pixel placementcharacteristics and said third set of pixel placement characteristics.9. The method of claim 8, wherein the first halftoning system is amatrix dot halftoning system.
 10. The method of claim 8, wherein thefirst halftoning system is a stochastic dot halftoning system.
 11. Themethod of claim 8, wherein the first halftoning system is a cluster dothalftoning system.
 12. The method of claim 8, wherein an upper grayscale limit of the transition region is equal to approximately onequarter of a total number of available grey levels.